With recent progress of LSI technology manufacturing apparatuses for effecting film making and etching under reduced pressure have been widely used in processes of manufacturer of LSIs and ultra LSIs. There are included as these apparatuses: these for RIE (reative ion etching). RF sputtering, RF bias sputtering, plasma CVD, ECR CVD, and ECR etching, including those such as ion sources in an ion implanting device, which utilize electrical discharge under reduced pressure, those for Si epitaxial growth, and those for CVD for various types, which utilize reactions under reduced pressure, and the like. These apparatuses employ the following various kinds of gases; Ar, Ne, O.sub.2, He, H.sub.2, encluding SiH.sub.4, BF.sub.3, PH.sub.3, SiCl.sub.2 H.sub.2, CCl.sub.4, CF.sub.4, BCl.sub.3, CH.sub.2 F.sub.2, and other reactive gases, to which apparatuses high purity gas not involving any impurity at all is needed to be supplied for improving the controllability of the concerning processes and characteristics of devices formed with use of these apparatuses.
At present, substantially satisfiable gases are available by the improvement of purifying techniques for raw gases, gas cylinders filled with gases, gas purifying devices, and piping materials and parts for various gases, etc.. That is, the present inventor has accomplished a gas supply system which is free from any leakage of gas to the outside, free from dead zones, and free from particles.
This system however suffers from a problem that impurities comprising moisture in the main mix into these gases through inner wall of pipe lines serving to feed these gases to individual process units. This problem has also been substantially solved by an improved technique to treat the surface of those pipe lines and development of instruments (regulators, pressure gages and mass flow controllers) in which gas retention parts are reduced to present particles from being produced valves, and joints, but with a difficulty left behind unsolved finally: moisture, H.sub.2 O adsorbed on the inner walls of the pipe lines.
In what follows, such a problem will be described, in which moisture is undesirably introduced into a chamber of the concerning device together with raw gases.
For example, mixing of H.sub.2 O into Ar gas to be introduced into a RF sputtering device which serves to make a film of metal such as aluminum causes a surface of a target of Al sputtered to be oxidized with ease by the H.sub.2 O involved in the atmosphere because of this surface being very active, for thereby permitting Al.sub.2 O.sub.3 (alumina) to be formed on this surface. Al.sub.2 O.sub.3 has a reduced rate of sputtering as compared with Al to reduce the rate of sputtering of a target and hence the speed of making a film is sharply lowered. In addition, such H.sub.2 O is incorporated into the resulting Al film to result in increased Al wiring resistance as well as reduced reliability to electromigration.
Moreover, introduction of H.sub.2 O into an RIE device causes O and OH groups, which are active in the plasma environment, to be produced, whereby in etching polysilicon for example SiO.sub.2 is formed on the etched surface to result in uneven etching and to prevent a selection ratio between the etched polysilicon and the underlying SiO.sub.2 film from being made sufficiently large. Such a problem was solved up to now by making use of baking that is by heating a gas pipe line to about 120 .degree. C. for thereby removing H.sub.2 O molecules adsorbed on the inner surface of the pipe line (this temperature 120.degree. C. is primarily specified by the upper limit of heat resistance of a filter material).
However, the technique described above can not provide a satisfactory result particularly for a gas supply system extending to such a device as operated under reduced the pressure: in a process under gas pressure ranging from about 1.times.10.sup.-4 to about 10.sup.-1 Torr, a flow rate of gas introduced into the device is reduced to thereby increase the degree of contamination due to gases chiefly composed of moisture and freed from the inner wall of a pipe line. Namely, with the flow rate more reduced, the degree of contamination is more increased.
In view of the drawbacks of the prior arts, it is an object of the present invention to provide a method of and a system for supplying ultrahigh purity gas capable of supplying ultrahigh purity gas to a device operated under reduced pressure.